Yes, do watch for any reaction although a bad one would probably have happened by now.
Years ago, had a Golden puppy get stung by a bee, ended up racing to the Vet's office. Dod was there for 3 days on IV's. :-(
I completely agree with Ginger. A call to your vet to make sure Benadryl is OK for very young puppies and to confirm what the correct dosage would be is in order. As far as the swelling goes, I would bet the rent that he just ran into a bee or a wasp or maybe even a spider, but bee or wasp is far more likely.
Keep an eye on him and as long as he isn't having a problem with breathing or eating, I would just wait it out and make sure the swelling goes down. It should go down by tomorrow, or at least a major part of it should. Please let us know how he does.
Ghilly
The dosage of Benadryl for dogs is quoted as 1mg per pound. It would be the right thing to use by the sound of it, as it does very much sound as if he was stung by a bee or wasp.
The only problem is I am unsure about the dosage -or whether Benadryl is OK or not -for very young puppies, like yours.
The best idea would be to call your vet, and ask his advice.